Ontogeny refers to the developmental course of an organism. Here we focus on ontogeny of behavior. In both humans and other animals, movements and sensorimotor integration together with reaction to external environmental stimuli can be measured several days or even several months before birth or hatching. Recent developments in epigenetics show that environmental stimuli experienced both by the parents and the embryo have long lasting effects for the next generations. Genetic determinism and early experience may shape individual behavior of animals leading to consistent inter-individual differences or personality profiles. Early life is sensitive to social learning, including imprinting and other forms of learning mediated by brain mechanisms, orchestrated by hormones and biological rhythms, and ending with sexual maturity. Behavioral changes can also occur during senescence and for instance old post-reproductive animals can play a significant role, such as the grandmother effect.